US3721744A - Electrically heated pit furnace,particularly for melting vitreous silica - Google Patents

Electrically heated pit furnace,particularly for melting vitreous silica Download PDF

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Publication number
US3721744A
US3721744A US00203574A US3721744DA US3721744A US 3721744 A US3721744 A US 3721744A US 00203574 A US00203574 A US 00203574A US 3721744D A US3721744D A US 3721744DA US 3721744 A US3721744 A US 3721744A
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US
United States
Prior art keywords
furnace
filaments
heating
heater
accordance
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
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US00203574A
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English (en)
Inventor
K Vatterodt
W Weiss
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Osram GmbH
Original Assignee
Patent Treuhand Gesellschaft fuer Elektrische Gluehlampen mbH
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    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B5/00—Melting in furnaces; Furnaces so far as specially adapted for glass manufacture
    • C03B5/02—Melting in furnaces; Furnaces so far as specially adapted for glass manufacture in electric furnaces, e.g. by dielectric heating
    • C03B5/033—Melting in furnaces; Furnaces so far as specially adapted for glass manufacture in electric furnaces, e.g. by dielectric heating by using resistance heaters above or in the glass bath, i.e. by indirect resistance heating
    • C03B5/0336—Shaft furnaces
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B3/00—Ohmic-resistance heating
    • H05B3/0019—Circuit arrangements
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B3/00—Ohmic-resistance heating
    • H05B3/62—Heating elements specially adapted for furnaces
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S65/00—Glass manufacturing
    • Y10S65/04—Electric heat

Definitions

  • heater wires arranged in a ring around the crucible are formed to have two different cross sections, one being thick to form connecting portions and the other thin, to form heating portions, the thin cross sections of the heater wires being arranged adjacent the zones to be heated, the thick connecting portions passing through the other zones; the heater wires for the various zones are commonly connected at one axial end to a common connection ring, and at the other axial end to separate connection rings, for separate energization in accordance with heating requirements.
  • the invention relates to pit furnaces, for example for melting vitreous silica or quartz-resembling glasses of high silica content, and more particularly to a pit furnace having an electrically heated crucible.
  • the furnace of the present invention is particularly useful for the continuous production of quartz tubing, where the raw material is filled in at the top of a cylindrical crucible and the molten vitreous silica is shaped to tubing in a nozzle, the nozzle forming the bottom of the crucible.
  • the tubing is discharged at the bottom.
  • the quartz tubing leaving the furnace can be seized by a drawing machine located below the furnace and be drawn out to receive its final shape.
  • the heater filaments of the crucible are commonly of a refractory metal such as molybdenum or tungsten and are usually arranged to form a ring within furnace, furnace surrounding the crucible symmetrically with respect to its axis.
  • a stream of protective gas such as hydrogen or a mixture of hydrogen and nitrogen is circulated around the heater filaments.
  • the continuous heater filaments are connected at the top and bottom ends to current supply rings and are jointly supplied with current.
  • This type of heater filament arrangement permits a reduction or increase in temperature only throughout the entire region of the crucible.
  • heating of the nozzle should be regulated independent of the crucible heating. In order to achieve such a heating of the nozzle independently of the crucible heating, the heater filaments were previously divided in the center so that an upper and a lower heater filament ring is obtained,
  • each ring being provided with an individual current supply which is independent of that of the other ring. Nevertheless it is difficult to design the heater filament arrangement so that no drop in temperature occurs at the transition point from the upper to the lower heater filament ring. Moreover, it is almost impossible to locate additional current supply conductors in the center, since they would melt due to the heat radiating from the furnace.
  • a ring of heater filaments is provided.
  • the heater filaments are rod-shaped and formed along their overall length such that they have connecting sections of thick diameter and heating sections of thin diameter.
  • the transition point between connecting section and heating section may be in any place of the heater filaments, e.g. at half their length.
  • the ring of heater filaments is arranged around the crucible such that the heater filaments for the heating of the upper furnace zone have thin upper heating sections and alternate with filaments which are axially reversed, that is, have the thin heating sections at their lower ends to heat the lower furnace zone independently of the other filaments.
  • the connecting sections of the heater filaments should contribute only little to the heating of the respective furnace zone, and their diameter is to be designed accordingly. The diameter of the heater filament sections contributing to heating depends on the furnace output required, which may vary depending on its use.
  • Both heater filament systems can be regulated electrically independently of each other.
  • Two electrically separated connecting rings are placed at the bottom end of the furnace.
  • the heater filaments are tightly screw-fitted into the connecting rings.
  • the upper one of these connecting rings has bores therein to pass the heater filament end portions for heating of the upper furnace zone therethrough, while insulated from the rings.
  • the upper connection is common for both systems.
  • the differential linear extension which occurs during start-up heating is absorbed by two tension systems which are capable of individually tensioning the filaments and include spring pockets to compensate for the linear expansion as disclosed in U.S. Pat. No. 2,998,469.
  • the heater filament system of the present invention permits adjustment of the temperature of both zones substantially independently of each other, so that the melting temperature in the crucible and the temperature of the nozzle become adjustable independently of each other. This if of particular importance when dimensioning quartz tubing and maintaining proper tolerances.
  • the set temperatures in both zones are affected only slightly by dissipation of Joule heat from the connecting sections, and the heat transfer due to the flushing gas.
  • FIG. 1 is a longitudinal sectional view of a pit furnace
  • FIG. 2 is a schematic circuit diagram of the furnace heating arrangement.
  • the electrically heated pit furnace is supplied at the top with the raw material which is melted to vitreous silica in the cylindrical crucible 1.
  • the vitreous silica is shaped to tubing by a nozzle 3 in bottom 2; with another appropriate nozzle design, cane of round or any other cross section can be formed.
  • the tubing discharged through the nozzle is seized and drawn out by a drawing machine until it is completely solidified.
  • the crucible is surrounded by a ring of continuous heater filaments.
  • the heater filaments are shaped, preferably up to about half their length, i.e.
  • Filament 4-5 is heavy at the bottom 5 and thin at the top, to heat the top portion but provide practically no, or only little heat to the bottom, or nozzle portion of the furnace; while filament 4'5' has a heavy connecting section on top and is dimensioned to heat, with section 4, the nozzle zone or portion of the furnace.
  • the number of heater filaments depends on the crucible diameter but in any case these must be even-numbered to provide an axially symmetrical arrangement of heater filaments with upper heating and lower connecting sections and vice versa (see FIG. 1).
  • the dimensioning of the diameter of the heater filaments depends first of all on the furnace output required, but the connecting sections should be dimensioned such as to contribute as little as possible to the heating of the furnace zone.
  • the two heater filament systems for the heating of the upper and the lower crucible zone can be controlled independent of each other, and by this means the temperature of the upper crucible zone can be adjusted independent of the lower zone. This is important particularly for the drawing procedure, as the nozzle temperature is of decisive importance for the dimensioning of the quartz tubing.
  • the temperature of the two zones is affected slightly by the dissipation of some Joule heat by the connecting sections, and also by the heat transfer by the flushing gas which surrounds the heater filaments.
  • the two heater filament systems are connected at the I bottom end of the furnace to two separate connecting Pat. No. 2,998,469).
  • a pit furnace constructed in accordance with FIG. 1 is rated for an electrical power requirement of 75 kva which is to be supplied to two heater filament systems,
  • the heater filaments are made from tungsten and have a connecting section of 8 mm in diameter and a heating wire section of 2.8 mm in diameter. (The heater filaments of both types are actually somewhat longer due to the length of the end portions which are clamp or screw connected; the clamps do not materially affect the calculation'of electrical data and are thus not included in the effective lengths given above).
  • each heating system can be loaded with 37.5 kva at the most. ln-case of 10 V terminal voltage, a maximum current of 3,750 A results, and with the given heater filament dimensions, a surface area loading of 80 W/cm ensues.
  • two transformer units 10, 10' (FIG. 2) of 60 kva each and of V secondary voltage are provided.
  • the excess capacity permits possible changes in the zone length or in the overall length of the furnace.
  • the transformer units l0, l0 consist each of a variableratio transformer connected as auto-transformers, with about 10 rough and eight fine adjustment steps in combination with a phase reversing switch 11, 11 and a two-core heavy-current transformer 12, 12'.
  • the heater sections of the filaments deliver approximately 90 percent of the total output and the connecting sec tions about 10 percent of the total output. This slightly additionally affects the temperature in both zones and leads to a blurring of the temperature maxima. The effect is of about the same magnitude as that produced by heat conduction and heat transfer.
  • Pit furnaces can be heated such as to havethree or even more independently regulable temperature zones.
  • the heater filaments are then designed accordingly;
  • the heater filaments are of such a design that either the upper, the central or the lower third of'their overall length is designed as a heating section, whereas the rest of the heater filament is designed as a thick connecting section.
  • the number of lower connecting rings as well as the number of rods forming the heater filament ring must be selected in accordance with the number of heating systems: with three temperature zones, three lower terminal rings are therefore required, and the total number of heaterfilaments must be divisible by three, in order to provide an axially symmetrical arrangement.
  • a circular drawing nozzle disposed in the bottom of the crucible for continuous extraction of tubing or cane
  • rod-like heater filaments are formed along their axial length to have a connecting section of thick diameter and a heating section of a diameter thin with respect to that of the connecting section to provide essentially all heat energy upon passage of current in the zone of the thin heating section; wherein an even number of said rod-like heater filaments are provided, alternately disposed heater filaments having heating sections alternately in the upper and in the lower portion of the filaments to form two filament systems; and means are provided individually electrically connecting the filament systems.
  • the means separately connecting the filament systems comprises two electrically separated connecting rings connected to the respective heater filaments and a common connecting ring located at the top of the furnace.
  • Furnace in accordance with claim 1 providing a melting zone of a first temperature and at least one more zone at a second temperature;
  • the filaments having the heating section in the upper portion forming a first heating system and being arranged to have the heating section thereof located axially adjacent one of the melting zones; and the filaments having the heating section in the lower portion forming another heating system being arranged to have the heating sections thereof located axially adjacent the other zone.
  • Furnace according to claim 8 including separately controllable current supply means connected to each said rings.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Organic Chemistry (AREA)
  • Resistance Heating (AREA)
  • Glass Melting And Manufacturing (AREA)
US00203574A 1970-12-30 1971-12-01 Electrically heated pit furnace,particularly for melting vitreous silica Expired - Lifetime US3721744A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
DE19702064433 DE2064433A1 (de) 1970-12-30 1970-12-30 Elektrisch beheizter Schachtofen zum Schmelzen von Quarzglas

Publications (1)

Publication Number Publication Date
US3721744A true US3721744A (en) 1973-03-20

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ID=5792555

Family Applications (1)

Application Number Title Priority Date Filing Date
US00203574A Expired - Lifetime US3721744A (en) 1970-12-30 1971-12-01 Electrically heated pit furnace,particularly for melting vitreous silica

Country Status (5)

Country Link
US (1) US3721744A (cg-RX-API-DMAC7.html)
DE (1) DE2064433A1 (cg-RX-API-DMAC7.html)
FR (1) FR2120138B1 (cg-RX-API-DMAC7.html)
GB (1) GB1342100A (cg-RX-API-DMAC7.html)
NL (1) NL7116139A (cg-RX-API-DMAC7.html)

Family Cites Families (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR708843A (fr) * 1931-01-05 1931-07-29 Dispositif réglable de chauffage, par résistance, de fours électriques pour traitements thermiques
GB637056A (en) * 1948-02-27 1950-05-10 Kenneth Charles Shotbolt Improvements in and relating to electrical heating for ovens with travelling conveyors
US2596327A (en) * 1949-07-19 1952-05-13 Shell Dev Electric heater
CH316954A (de) * 1952-09-01 1956-10-31 Siemens Ag Drehstrom-Hochvakuumofen
FR1491108A (fr) * 1966-09-02 1967-08-04 Australlian Atomic Energy Comm Fours à résistance électrique pour le chauffage à des températures élevées

Also Published As

Publication number Publication date
FR2120138B1 (cg-RX-API-DMAC7.html) 1975-07-18
NL7116139A (cg-RX-API-DMAC7.html) 1972-07-04
GB1342100A (en) 1973-12-25
FR2120138A1 (cg-RX-API-DMAC7.html) 1972-08-11
DE2064433A1 (de) 1972-07-20

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